Computer control of paper machine in which basis weight is controlled through control of stock flow

ABSTRACT

Apparatus is disclosed for controlling the basis weight of double-layer paper from a Fourdrinier machine by: comprising composite dry basis weight from total basis weight and moisture measurements; computing the dry basis weight of the top layer from the composite dry basis weight and the stock flow and consistency data of both layers; correcting the stock flow rate for the top layer to achieve a desired top layer dry basis weight value; and correcting the stock flow for the bottom layer to achieve a desired composite dry basis weight taking into consideration the correction made in the stock flow of the top layer. Composite dry basis weight changes due to changes in machine speed and stock consistency are anticipated and corrected by making a compensating change in stock flow for both layers. The flow rate of fiber onto the wire screen of the machine is controlled by computing headbox total flow setpoints from fiber flow data and desired headbox consistency, and adjusting the headbox slice position accordingly. Speed of the slice jet relative to the wire screen is controlled by controlling headbox head.

United States Patent Stout et a1.

BLQ

[ COMPUTER CONTROL OF PAPER MACHINE m WHICH BASIS WEIGHT IS CONTROLLED THROUGH CONTROL OF STOCK FLOW [75] Inventors: Thomas M. Sto t Noithridge, Califl; Edward J smith, Houston, Tex.; John 111. Hiestand, Canoga Park, Calif. [73] Assignee: The Bunker-Rama Corporation Canoga Park, Calif. 22 Filed: June 27,1968

21 Appl.No.: 740,731

[52] US. Cl. ..235/l51.1, 162/259, 444/1 [51] Int. Cl. ..D2lf 1/06 [58] Field0fSearch.235/l51.1,151, 151.13, 151.35, 235/150, 150.1; 162/252, 254, 258, 262, 263, 253, 259; 73/73 X [56] References Cited UNITED STATES PATENTS 3,490,689 1/1970 Hart ct a1. ..235/l5l.1 2,540,301 2/1951 Staege "162/259 3,077,924 2/1963 Eastwood ..l62/259 3,135,652 6/1964 Smith ..162/259 3,165,439 1/1965 Lejeuneetal. .....l62/259X 3,271,241 /1966 Mumme .,162/259X SPTLFL TLO TLFL 54 SPBLFL S AC-l HEAD B BOTFOM L AYETZ I Jan. 16, 1973 3,293,120 12/1966 Harman, Jr. et al ..162/253 3,461,030 8/1969 Keyes ..162/252 X Primary Examiner-loseph F, Ruggiero Attorney-Frederick M. Arbuckle [57] ABSTRACT Apparatus is disclosed for controlling the basis weight of double-layer paper from a Fourdrinier machine by: comprising composite dry basis weight from total basis weight and moisture measurements; computing the dry basis weight of the top layer from the composite dry basis weight and the stock flow and consistency data of both layers; correcting the stock flow rate for the top layer to achieve a desired top layer dry basis weight value; and correcting the stock flow for the bottom layer to achieve a desired composite dry basis weight taking into consideration the correction made in the stock flow of the top layer. Composite dry basis weight changes due to changes in machine speed and stock consistency are anticipated and corrected by making a compensating change in stock flow for both layers. The flow rate of fiber onto the wire screen of the machine is controlled by computing headbox total flow setpoints from fiber flow data and desired headbox consistency, and adjusting the headbox slice position accordingly. Speed of the slice jet relative to the wire screen is controlled by controlling headbox head.

38 Claims, 6 Drawing Figures BLHD BLSL BLHDl N SPBL 52 TOP LAYER '20 HEAD BOX PATENTEUJAH 16 I973 SHEET 3 OF 6 \NVHAUZE FIRST axacunow ONLY; THEREAFTER DATA UPDATED BY TH\$ PRoeRAM SPDOt- SPDN BLCO BLCN T LCO-- T LCN lbPDN-SPDO REPEAT EVERY $5 SECONDS v ON CALL BY DC-Z COMPuTE 5, UPDATE:

SPDN SPDO BPDN BLCN BLCOI :Q2

COMPUTE 2 UPDATE;

BLCN BLCO B LCJN COMPUTE ITLCN TLC.O 2 Q 5 COMPUTE &. UPDATE:

TLCN TLCC) T LCN T LCO T LCN OMPLATE TLFL(D,-D3)

COMPUTE $ETPOKNTE 0O) OUTPUT NEW 9ETPOI NTS TO AC-\ E, AC-4 THOMAS M. STOUT fDl/VAPD SM/TH JOHN H H/ESTAND A TTOP/VEYS PATENTEDJANISISH 3.711.688

saw u or 5 coMPuTE PRE$ENT Pogmow. DBW =(|oo-/v\o BW BLFF =(BL'FLxBLc) Q TLFF =(TLFL XTLC )Q TOTFF (TL FF BLF F) TLDBW DBW Tl-FF TOTFF COMPOSITE (015w) AND COMPMTE ERRORS TOP LAYER (TLDBW) ERRORS ERDBW SPDBW DEW FORTL ANB BL FLOW ERTLDBW= SPTLDBW-TLDBW CONTROL SETPCNNT \NCREMENTs COMPUTE FLOW SEX-Down. \NCREMENTS ADDED TO CLORR 5p \NCREMENTS or- Tu=1 1% TLFL DC-l PROGRAM AT T END OF- THIS nc-a BLFIZ lEfl Lc. TLF12 QOCaRAM DEW BLC V CALL up (4) EXECUTE DC-\ PROGQAM /NVE/\/7O/?S THO/HA5 M. sTour EDWARD J. -.5M/7"H COMPUTER CONTROL OF PAPER MACHINE IN WHICH BASIS WEIGHT IS CONTROLLED THROUGH CONTROL OF STOCK FLOW BACKGROUND OF THE INVENTION 1 Field of the Invention This invention relates to computer controlled processes and more particularly to a system for using a computer to so control a Fourdrinier paper machine as 1 to maintain the basis weight of paper being produced at a desired level.

2. Description of the Prior Art Basis weight is probably the most important characteristic of paper, since nearly all paper and much of the paperboard is sold on a specified basis weight. Basis weight is the weight per unit area of paper or paperboard. It can be measured by a commercial instrument as the product is taken off the machine and wound on a reel. The instrument customarily used is a B-ray gauge. As measured, basis weight reflects the total weight of the product, including about 7.5 percent moisture if the paper has been equilibrated, i.e., has reached a point at which it will neither absorb nor surrender moisture from or to the atmosphere. In the past, desired basis weight and other paper characteristics have been achieved largely by intuitive control of such factors as stock flow. Although some thought has been given to computer control, the strategy for control suggested has been largely to reproduce the actions of the operators. Although such a strategy would lead to more consistent results, much would be left to be desired, since the operators strategy is limited to dependent variables that may be readily measured or quickly calculated.

Basis weight feedback control, whether done manually or automatically, presents a number of problems when based only upon basis weight measurements on the paper reel because factors which have an effect on basis weight may vary widely, sometimes purposely, and the change in basis weight will not be detected for some time due to the finite time required for stock to pass through the machine and appear as paper. For instance, to achieve maximum production at all times, frequent changes in machine speed may be desirable. However, in the past machine speed has not been changed as frequently as desirable because of the upsetting effect it has on other machine settings, such as stock flow rate. It would be desirable to have stock flow adjusted in anticipation of any change in basis weight due to machine speed changes and other changes, such as consistency of stock to the headbox.

No control system would ever really be complete without some feedback control regardless of what anticipatory (feed forward) control is provided. But feedback control based on measurements of basis weight at the paper reel would not be totally satisfactory in a double-layer machiner, where it is desirable to control both the composite basis weight of top and bottom layers and the basis weight of just one of the layers. Therefore it would be desirable to be able to control the basis weight of the one layer independently of the composite basis weight, and the composite weight as a function of the desired composite weight and the con-' trolled basis-weight of the one layer.

Although it is customary to regulate stock flow to the machine headbox for control of basis weight, and to maintain consistency of the stock through the flow regulator as constant as possible, there are still significant variations experienced in the stock flowing from the headbox onto the wire screen of the machine for various reasons. The presently available consistency regulators are not capable of controlling consistency very closely at the low levels involved (3.0 to 3.5 percent). Moreover, the stock is further diluted to a consistency of about 0.5 percent downstream from the consistency regulator and the stock flow control valve, i.e.,just before the stock enters the headbox. The stock is not diluted to such a very low level of consistency at the inlet of the consistency regulator because it is incapable of regulating at that low a level. Dilution is accomplished with water from a silo flowing at a set rate. If thick stock flow remains constant both as to flow rate I and consistency, the consistency of the diluted stock in the headbox remains constant, but flow rate and consistency of the thick stock may vary.

Headbox consistency (about 0.5 percent) affects fiber retention on the wire screen, and therefore affects the dynamics stability of basis weight, a paper characteristic that is more dependent on thick stock (3.0 to 3.5 percent) flow and wire screen speed. Therefore, to prevent transient variations of basis weight due to a varying fiber retention, it would be desirable to be able to compensate for variations in headbox consistency based upon some indirect measurement of it, since it cannot be measured directly. However, even is some indirect measurement of headbox consistency is made, the problem of compensating for it remains, since the stock flows directly from the headbox to the wire screen where the paper is formed. One could, presumably, control the flow of diluting water on the basis of the indirect measurements to directly control consistency of diluted stock in the headbox, but that would not be entirely satisfactory. It would be desirable to employ some more immediate compensation for variations in the consistency of stock as it passes out of the headbox slice onto the wire screen, as by adjusting the slice opening. It is also desirable to control the relative speed of stock from the slice and the moving wire screen. That is customarily done by adjusting the slice opening. If the slice opening is adjusted to compensate for variations in headbox stock consistency, other means must be found for controlling stock velocity at the slice.

OBJECTS AND SUMMARY OF THE INVENTION An object of this invention is to provide a system whereby the basis weight of paper is controlled directly by the control of stock flow.

Another object is to provide a system whereby the composite basis weight of two-layer paper is controlled while the basis weight of one layer is independently controlled.

Another object is to provide a system whereby variations in the consistency of stock in a headbox is compensated for by the control of slice position.

Yet another object is to provide a system whereby the velocity of stock flowing out of the headbox relative to the wire screen is directly controlled by the control of pressure of stock at the slice.

These and other'objects are achieved in an arrangement wherein signals developed by sensors or transducers are applied to a computer adapted to receive such signals as data concerning the process being controlledv and having provided therein four computing sections, preferably in the form of stored programs, for performing certain mathematical computations on the data to provide control functions. The first control function is to so control stock flow as to anticipate changes in paper basis weight resulting from changes in factors which will affect basis weight, such as machine speed and stock consistency. Anticipated changes in basis weight are compensated for by adjusting the flow rate of stock. The second control function is to maintain basis weight at a desired setpoint by first measuring total basis weight and subtracting measured moisture. The computed dry basis weight is then compared with a setpoint to generate a control signal to a stock flow valve. In a two-layer paper machine, where the basis weight of both the composite paper and one of the layers thereof is to be maintained at a desired setpoint, the dry basis weight of the one layer is calculated from a ratio of its dry basis weight to the dry basis weight of the composite paper in proportion to the ratio of its fiber flow to the sum of fiber flow rates to both layers. Each fiber flow rate is computed from its measured stock flow rate and the measured consistency of its stock going into its control valve. The third control function is to compensate for variations in the consistency of diluted stock in the headbox (to maintain a constant fiber flow for a given machine speed and desired basis weight) by varying the slice position. Since actual headbox consistency cannot be measured, it is calculated as a function of flow rate and consistency of stock to the headbox, and controlled by setting the headbox flow in proportion to the ratio of the computed headbox consistency to a desired headbox consistency. Thus, headbox flow is controlled by the slice position. The fourth control function is to maintain desired rush or drag of the stock from the slice by adjusting the head (headbox pressure). To accomplish that, the wire speed of the machine is measured and a theoretical head is computed from it. The operator then effectively adds or subtracts from the theoretical head an amount predetermined to produce the desired rush or drag. A controller then maintains the head at that pressure until there is a change in speed, at which time a new theoretical head is computed and a new setpoint is determined for the controller.

The novel features of the invention are set forth with particularity in the appended claims. The invention will best be understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a typical twolayered paper mill to be controlled with a digital computer in accordance with the present invention;

FIG. 2 is a block diagram of a computer installation in a paper mill in accordance with a preferred embodiment of the invention; and

FIGS. 3 to 6 illustrate flow charts of four sections of control to be exercised in a paper mill in accordance with the preferred embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 illustrates in a simplified form a typical installation of a Fourdrinier paper machine.'Dry material is first reduced to pulp and refined to a consistency of about 3.5 to 4 percent fiber in water by means not shown and then deposited in a machine chest 10. A fan pump 11 transfers thick stock to a headbox 12 via a consistency regulator 13 and a headbox level controller 14. The latter may consist of another fan pump with a recirculating line having a control valve adjusted by analog or digital computational means in accordance with the difference between a desired level and the actual level as indicated by a signal transmitted over a line 15. A valve 16 between the consistency regulator 13 and the headbox level controller 14 is provided to set the desired flow rate of thick stock.

The headbox 12 may be either of the open or closed type. As will be noted hereinafter, in a preferred embodiment of this invention actually reduced to practice, the headboxes were of the closed type employing level controllers for maintaining stock at a substantially constant level and pressure regulators for maintaining the air space above the stock at a desired pressure. In that manner, headbox pressure is controlled independently of stock level. In an installation employing open headboxes, the level controller would be relied upon to maintain the head at the desired value.

The consistency of stock furnished to the headbox 12 should be about 0.5 percent. The consistency regulator 13 dilutes the machine chest stock to a consistency of about 3.0 to 3.5 percent with water (which may be taken from a silo 17). A valve 18 is adjusted by the consistency regulator 13 via line 19 for that purpose. The thick stock is then further diluted by introducing about six parts of water to one part of stock at a point between the flow control valve l6and the headbox level controller 14, usually at the fan pump of the latter. In that manner the stock in the headbox 12 is maintained at about 0.5 percent.

An operator manually sets the control valve 16 and the opening of a headbox slice 20. The latter has, in the past, been set to a desired rush or drag such that the stock passes on to an endless moving wire screen 21 at a velocity greater or less than the motion of the wire screen in order to achieve a desired fiber orientation on the screen. In the present invention, the slice position is adjusted to compensate for variations in the consistency of stock in the machine. Desired rush or drag is then maintained by controlling the headbox pressure (head).

The screen burdened with stock is supported by a forming table 22. From there the screen passes over suction boxes 23 where water is drawn out and transferred into the silo 17. leaving a damp fiber web on the surface. The web is then removed as paper from a couch roll 24, usually a hollow perforated roll with a vacuum chamber 25 inside. A pump 26 provides the vacuum for the suction boxes 23 and the vacuum chamber 25. It is usually a wet vacuum pump, and the water it draws is then returned to the silo 17, or otherwise returned to the system.

After the paper is removed from thevscreen, it is passed through press rollers 27, a drying section 28 and onto a reel 29. The basis weight and moisture of the paper is generally measured by on-line transducers between the drying section and the reel. Other tests are also made there, such as bursting strength. It is on the basis of those tests that the paper is graded. Feedback control of basis weight is achieved on the basis of those and other machine measurements in accordance with the present invention.

The simplified machine thus far described is conventional for the production of paper having a substantially uniform basis weight throughout its thickness as well as length. For some applications, such as paperboard containers, it is customary to have a bottom layer of one basis weight and a top layer of higher basis weight. To achieve such dual-layer paper production, a second headbox system identical to the one just described is provided with its slice downstream from the slice of the first system, as shown. Since the systems are the same, but operated with different adjustments to achieve the desired result, the second system will not be described except, with reference to FIGS. 2 through 6, as to how it is controlled relative to the first in accordance with the present invention. However, to facilitate understanding the environment of the present invention, as disclosed in FIG. 1, reference numerals are assigned to components of the second system, and the same reference numerals for both systems are then employed in FIG. 2.

Referring now to FIG. 2, a computer 40 comprising four sections DC-l DC-2, DC-3 and DC-4 is provided to control the paper machine in accordance with the present invention. Each computer section will be separately described in turn. It should be understood that in the preferred embodiment each is a part ofa single digital computer, or more particularly a separately stored program, or section of a program, called into operation periodically in the case of sections DC-l and DC-4 and on demand in the case of sections DC-2 and DC-3. However, separate special purpose computing sections could be provided if that be, or become, economically feasible without departing from the true spirit and scope of the invention. Indeed, some or all of the functions of the computer 40 may be provided by analog computational means, although digital computational means are to be preferred for the various control functions because of inherently greater accuracy at less cost and greater flexibility when making changes in the system. Therefore, such computational means comprising the present invention must be construed to embrace both digital and analog means and, for purposes of defining the invention, are to be regarded as separate means for each step described or a common means suitably adapted by a stored program for different steps described.

Before proceeding with a detailed description of the various sections of the computer 40, it should be noted that an embodiment of the present invention was reduced to practice with a commercial computer BR- 340, which has a core memory used for all operations, including input and output operations, and an auxiliary magnetic drum memory for storing programs, constants to be used in the programs, and data to be carried forward from one execution of a given program to the next or to the execution of another program. However, other commercial computers adapted for analog input and output operations may be employed.

DC-l SEQTION The first section DC-l anticipates basis weight changes caused by a change in machine speed or a change in stock consistency from regulators l3 and 33 to respective headboxes l2 and 32. If a change from previous values in speed or consistency greater than a fixed deadband has taken place, the stock flow to the fan pumps in the headbox-level controllers l4 and 34 is adjusted in proportion to the change. At the same time, the previous values (deadband centers) are set to the then present speed and consistency values.

The control output of the section DC-l is a new setpoint to a bottom layer flow controller AC-I which adjusts the flow valve 16 and a new setpoint to the top layer flow controller AC-2 which adjusts the flow valve 36. The program is repeated every 15 seconds so that bottom layer and top layer stock flow is continually being adjusted to compensate for changes in machine speed and consistency.

The consistencies of the bottom layer and top layer stocks are measured by the respective consistency regulators l3 and 33. Thus, present bottom layer consistency (BLC) and top layer consistency (TLC) signals are continuously transmitted from the consistency regulating feedback lines 19 and 39 to the sec tion DC-l as well as sections DC-2 and DC-3 via lines identified in FIG. 2 by the codes BLC and TLC. The other input to the section DC-l is speed which may be a new speed setting since the last time the program was run. That speed input (SPDN) is entered at the console of the computer represented by a terminal SPDN at the same time that the machine operator makes a change in the machine speed. The computer 40 converts these and other input signals into a suitable form by, for example, a time-stored analog-to-digital converter in the input/output section thereof too well known to those skilled in the art to require its description here.

There is a transducer 4] provided to continuously measure the speed of the wire screen 21. The output of that transducer is transmitted to the computer section DC-4 as an input signal (SPEED) viaa line SPEED. However, that signal is not applied to the terminal SPDN since the object of the DC-l program is to anticipate basis weight changes caused by machine speed changes, and not to correct for changes in speed detected by the transducer 41. The feed-forward, basis weight control provided by the DC-l section will enable the operator to make more frequent changes in machine speed in order to take advantage of all the available machine capacity. Without it, operators are more reluctant to make speed changes in the machine because of the effect it will have on basis weight. In other words, in the past the operator has been required to manually adjust speed to meet particular operating requirements as well as other machine variablescontrolled by this and other sections of the computer 40.

The feed-forward, basisweight control program of the section DC-l will now be described with reference to the flow chart of FIG. 3. A detailed program (automatically repeated every 15 seconds) may be readily devised for a particular computer to implement the control equations. Accordingly, only a general control flow chart is shown in which the collection of blocks, lines, arrows and comments taken together indicate what functions are to be performed just as a general block diagram of an electronic system indicates functions performed in a particular organization to achieve a desired result. To avoid any misunderstanding, a somewhat standardized system of symbols has been employed. A block with square corners indicates an operation, or group of operations, and a block with round corners indicates a logical decision to be made on the basis of a test or comparison indicated. Large circles are employed to indicate the beginning (START) and the end (EXIT). Each major step is numbered in sequence in parentheses next to theoperation or decision block involved. The control equations are:

TLFl =TLFL(D,D (2) [SPBLFL] =SPBLFL +BLFl +BLFI (3) [SPTLFL] =SPTLFL+TLFI +TLFI 4) where the various terms are defined in the following manner:

BLFI a signal developed by computing means of section DC-l for incremental change in bottom layer flow to be made through controller AC-l.

BLFL a signal representing a present bottom layer stock flow developed by a sensor and applied to computing means of section DC-l.

TLFI a signal developed by computing means of section DC-l for incremental change in the top layer of flow to be made through controller AC-2.

TLFL a signal representing a present top layer stock flow developed by a sensor and applied to computing means of section DC-l.

BLFI a signal developed by computing means of section DC-2 for incremental change in bottom layer flow to be made through controller AC-l.

TLFl a signal developed by computing means of section DC-2 for incremental change in top layer flow to be made through controller AC-Z.

SPBLFL bottom layer flow setpoint signal for controller AC- l.

SPTLFL top layer flow setpoint signal for controller AC-2.

D a signal representing a value proportionate to any change made in machine speed since the last time the program was executed.

D a signal representing a value proportionate to any change in bottom layer consistency BLC measured by a sensor in the consistency regulator 13 (FIG. 2) since the last time the program was executed.

D a signal representing a value proportionate to any change in top layer consistency TCL as measured by a sensor in the consistency regulator 33 (FIG. 2) since the last time the program was executed.

The present thick stock flow signals BLFL and TLFL are provided by flow meters 42 and 43 at the output of the respective flow meters 42 and 43 at the output of the respective flow control valves 16 and 36. Those signals are applied directly to controllers AC-l and AC-2 for comparison with setpoints transmitted thereto from the computer section DC-i. The controllers then adjust the flow valves 16 and 36 until the difference between the present flow signals and the respective setpoint signals is each reduced to zero. The

analog signals representing the present flow values BLFL and TLFL are also transmitted to the DC-l section as well as the DC-2 and DC-3 sections of the computer 40 via an input/output section thereof.

The first time the program of section DC-l is executed (once the paper machine is placed under com-;

puter control), three values stored in memory are initially set. Those values are:

SPDO previous speed set equal to the present speed SPDN manually, or otherwise, entered into computer section DC-l.

BLCO previous bottom layer consistency set equal to present bottom layer consistency BLCN as represented by the signal BLC at the time.

TLCO previous top layer consistency set equal to present top layer consistency TLCN as represented by the signal TLC at the time.

The first step, once it is established that the paper machine is under program control, is to determine whether the absolute value of difference between the present speed and the previous speed is less than a deadband level Q (which may be experimentally set at anywhere between zero and 20 for wire speeds of up to about 1,800 feet per minute, starting with O equal to 10). If so, D, is set equal to zero, and the second step is skipped. In the second step, the value D, is computed as the ratio of the present speed less the previous speed to the present speed. Once that ratio has been computed, the present speed SPDN is substituted for the previous speed SPDO in memory for use the next time the program is executed.

in the third step, the absolute value of the difference between the present bottom layer consistency and the previous bottom layer consistency is compared with a deadband level 0 (where that level may be experimentally set between 0 and 0.1 starting with 0.02 for a consistency range of 0 to 1 percent). If that difference is less than 0 the value D is set equal to O and the next step is skipped. Otherwise, in the next step the value D is computed as the ratio of the present bottom layer consistency less the previous bottom layer consistency to the present bottom layer consistency. Once that value D, has been computed, the value of the present bottom layer consistency signal BLCN is substituted for the value of the previous bottom layer consistency signal BLCO in memory for use the next time the program is executed. in the fifth step, the incremental change signal BLFl to be made on the setpoint signal SPBLFL for the bottom layer flow controller AC-l is computed in accordance with the foregoing equation 1.

In the sixth, seventh and eighth steps, the procedure followed for the bottom layer flow in steps 3, 4, and 5 is repeated for the top layer flow to compute the incremental change signal TLFI to be made in the setpoint signal SPTLFL for the top layer flow controller AC-2. In the ninth step, the increment signals BLFI and TLFI are added to the respective setpoint signals in accordance with the foregoing equations 3 and 4, where the brackets signify the new setpoint signals to be transmitted to the respective controllers and substituted in memory for the present setpoints. At the same time, incremental change signals BLFI, and TLFI, computed by the computer section DC-2 are added to the respective setpoint signals.

From the foregoing, it may be seen that, as speed is increased or consistency is decreased for either the bottom layer or the top layer, stock flow will be increased in order to maintain the rate at which fiber is furnished to the paper machine more nearly constant. Otherwise, the basis weight of the paper produced will be reduced. If the change in speed or consistency is in the opposite direction, flow is decreased. Thus, the last step of the program is to output the new setpoint signals for the controllers AC-l and AC2 to effect a compensating change in stock flow. The manner in which that is done through the input/output section of the computer is too well known to those skilled in the art to require a description.

DC-2 SECTION Referring again to FIG. 2, the DC-2 section provides feedback control to maintain the dry basis weight of the top layer at a setpoint SPTLDBW and the composite dry basis weight DBW of the top layer and the bottom layer at a setpoint SPDBW. The program consists of two parts. In the first part, the present composite dry basis weight DBW is calculated from a measured total basis weight BW and moisture MO of the paper going onto the reel 29. Transducers 44 and 45 are provided to make those measurements. Transducer 44 is a B-ray gauge which scans the paper as it is fed onto the reel 29 starting on one edge and scanning diagonally across to the other edge and then back diagonally across to the starting edge. One scanning cycle takes approximately 3.25 minutes. During that cycle, the analog signal from the transducer 45 is sampled by the computer 40 at short intervals, and the average of all samples taken during a cycle is computed as the total basis weight BW. An average moisture measurement MO is similarly obtained with a scanning transducer 44 mounted on the same scanning mechanism employed for the B-ray gauge. The moisture transducer 44 may consist of a Wheatstone bridge with the paper between two slanting contacts constituting one branch of the bridge.

Once the basis weight scanning cycle has been completed, the preliminary calculations in the first part of the DC-Z program (FIG. 4) are made in accordance with the following equations:

DBW=(l00-MO) BW/lOO (5) BLFF=(BLFL X BLC)C (6) TLFF (TLFL X TLC)C TOTFF TLFF BLFF C, a coeff cient for converting gallons per minute to tons per hour.

TOTFF total fiber flow.

TLDBW top layer dry basis weight.

In that manner, composite dry basis weight is computed from the total basis weight of the paper going onto the reel, including its moisture content, where moisture is defined in terms of percent of total basis weight. The top layer dry basis weight is then calculated from the ratio of the top layer fiber flow to the total fiber flow, where the top layer fiber flow is calculated from the top layer stock flow signal TLFL and its consistency signal TLC through the flow control valve 36. The bottom layer fiber flow is similarly calculated to compute total fiber flow TOTFF by adding the top and bottom layer fiber flows TLFF and BLFF.

Once the present position calculations have been completed in step 1 of the flow chart of FlG. 4, the next step is to compute errors in composite dry basis weight and top layer dry basis weight in accordance with the following equations:

ERDBW SPDBW DBW (l0) ERTLDBW SPTLDBW TLDBW (II) where the setpoint for the composite dry basis weight (SPDBW) and the setpoint for the top layer dry basis weight (SPTLDBW) are entered by the operator at the console as represented by terminals in FIG. 2 identified by the respective legends SPDBW and SPTLDBW.

Once the errors in dry basis weight have been separately computed for the composite paper (top and bottom layers combined) and the top layer, the next step is to compute increments to be added to the bottom layer flow setpoint SPBLFL and the top layer flow setpoint SPTLFL in accordance with the following equations:

TLFl (ERTLDBW)/(TLDBW) TLFL (l2) BLFl (ERDBW)/(DBW) BLFL (TLC)/(BLC) TLFI 13) where top layer flow TLFL and bottom layer flow BLFL are present flow rate signals developed by the respective top layer flow meter 43 and bottom layer flow meter 42. The top layer flow increment signal TLFl is computed first from the top layer flow signal in proportion to the ratio of the error in the top layer dry basis weight to the setpoint for the top layer basis weight. The bottom layer flow increment signal BLFl is similarly computed from the bottom layer flow signal in proportion to the ratio of the composite dry basis weight error to the computed composite dry basis weight less that signal representing the increment of stock flow computed for the top layer in proportion to the ratio ofthe top layer consistency signal to the bottom layer consistency signal. In that manner, the specified top layer dry basis weight is maintained substantially constant while the specified composite dry basis weight is also maintained substantially constant through the control of the top and bottom layer stock flow rates. Since the top layer characteristic is more important, the feedback control program of the computer section DC-2 calculates the incremental change on the top layer flow rate and then adjusts the bottom layer flow rate to maintain the desired composite dry basis weight, taking into consideration changes being made in the top layer flow rate to maintain the top layer basis weight substantially constant.

Once the top and bottom layer flow increment signals have been computed, the values of the incremental signals TLFl and BLFl are added to the respective setpoints SPBLFL and SPTLFL by calling up and executing the program of the section DC-l. In that manner, the incremental feedback control signals are transmitted to the stock flow controllers AC-l and AC-2 through the program of the computer section DC-l as indicated in FIGS. 2, 3, and 4.

Since the DC-l program is repeated every seconds and the DC-2 program about every 3.25 minutes, it may be desirable not to require increments from the DC-2 program to be added to the setpoints in accordance with equations 3 and 4 each time the DC-l program is executed, but to instead modify the DC-2 program to include in step 4 the adding of values representing the incremental signals BLFl and TLFl to the respective setpoints SPBLFL and SPTLFL in memory without transmitting the new setpoints to the controllers. Instead, the new setpoints would be substituted for the previous setpoints in the specified memory locations addressed by the DC-] program to read the setpoints to which the incremental signals BLFl and TLFI are added. The DC-l program would them transmit signals representing the new setpoints to the controller.

DC-3 SECTlON The program of the DC-3 section also consists of two parts. In the first part (step 1 of FIG. 5), certain present position calculations are made. The results of those calculations are then employed in the second part (steps 2, 3, and 4) to compensate for variations in bottom layer and top layer headbox consistency by computing for each headbox total flow setpoints based on fiber flow rates and headbox consistency setpoints SPBLHBC and SPTLHBC entered by the operator at the computer console as represented by terminals in FIG. 2 identified by the legends SPBLHBC and SPTLHBC. Other input signals to the DC-3 section are: bottom layer consistency BLC from the consistency regulator 13; top layer consistency TLC from the consistency regulator 33; top layer flow rate TLFL from flow meter 43; bottom layer flow rate BLFL fr-om flow meter 42; bottom layer head BLHD from headbox 12; bottom layer slice position BLSL from a controller 46; top layer head from the headbox 32; and top layer slice position TLSL from a controller 47.

As just noted, the object of the program for section DC-3 is to control headbox flow by positioning the slice thereof in order to compensate for variations in headbox consistency. However, neither headbox flow nor headbox consistency can be measured directly. Accordingly, the first step of the program is devoted to computing for each of the headboxes the headbox flow based on its slice position and head. To introduce a compensation for variation in headbox consistency, a setpoint for the flow of stock from each of the headboxes is computed from the flow of stock through its respective stock flow meter, and the ratio of the con sistency of that stock (measured by its respective consistency regulator less a predetermined correction factor) to its headbox consistency setpoint (less a predetermined constant). Thus, the preliminary calcu-r lations made are in accordance with the following equations:

BLHBF (BLSL BLHD"")C 14 TLHBF (TLSL TLHD"")C SPBLHBF= (BLC E)BLFL SPBLHBC F. (16) SPTLHBF (TLC F) TLFL SPTLHBC F 17 where: C is a coefficient for converting inches to gallons per minute for the bottom layer headbox; C is a coefficient for converting inches to gallons per minute for the top layer headbox; E and F are factors included to correct for drainage fiber left in the recirculating water from the silo. Both factors may be substantially the same or different, depending upon the particular arrangement provided for returning drainage water from the machine to the system. Both equations 16 and 17 are derived from material flow equations which require that the product of headbox consistency and flow rate of diluted stock (about 0.5 percent consistency) equal the thick stock consistency and its flow rate plus the product of the correction factor and the difference between the headbox flow rate and the thick stock flow rate, since that difference is the water added from the silo to reduce the thick stock consistency from about 3.0 to 3.5 percent fiber to the desired headbox consistency of about 0.5 percent fiber.

Following the preliminary calculations, any error in flow from each of the headboxes is calculated in accordance with the following equations:

However, before those errors are computed, the corresponding errors computed the last time the program was executed are first transferred to specified memory locations for recall as previous errors PERBLHBF and PERTLHBF. From the error just computed for each headbox and the corresponding previous errors, a slice position increment signal BLSPl for the bottom layer headbox and a slice position increment signal TLSPl for the top layer headbox are then computed in accordance with the following equations:

BLSPl BLSL[G(ERBLHBFBLHBF) +H(PERBLHBFBLHBF)l (20) TLSPl TLSL[ I(ERBLHBFTLHBF) +J( PERTLHBFTLHBF)] (21) where G and I are each one of two constants for a twomode digital control algorithm and H and .l are each the other of the two constants for a two-mode digital control algorithm. These constants are first determined from a mathematical model of the control system for the headboxes l2 and 32 and then adjusted (tuned) for the particular characteristics of the respective headboxes in a manner well known to those skilled in the art of digital control of industrial processes. Briefly, it can be shown that for a three-mode analog algorithm of the form K,., K, and K are respective constants of proportional control, integral control and derivative control, and that a corresponding three-mode digital algorithm may be derived in the form AP: A B|' CGFQ where the terms of both equations are defined as follows:

P continuously manipulated variable.

AP increment of manipulated variable.

6 continuously varying error.

e,- present error.

e,- previous error.

e error previous to e,- The three constants A, B, and C taken as a group, not individually, correspond to the three constants K K, and K,, as follows:

The analog algorithm constants are quite difficult to adjust in an analog controller, but not in a digital controller, since each may be set equal to any precise value in a digital computer.

For a two-mode control algorithm using only proportionaland integral terms, the digital algorithm is of the form In the foregoing equations for the presently manipulated variables BLSBl and TLSBl, the constants tuned for the headboxes are identified as G and H in the case of the headbox l2, and l and J in the case of the headbox 32 instead of A and B for the control algorithm of general form, while the error e is, of course, ERBLHBF for the headbox 12 and ERTLHBF for the headbox 32. The error e,- is PERBLHBF for the headbox l2 and PERTLH BF for the headbox 32.

The increments BLSPl and TLSPl are employed in step 3 to compute new setpoints for the slice of the headbox l2 and the slice 39 of the headbox 32 in aceordance with the respective following equations:

[SPBLS SPBLS BLSPl [SPTLS ]=SPTLS +TLSPI 31) The new setpoints set off by brackets in the equation are signals transmitted to controllers 46 and 47 in step 4 to adjust the positions of the respective slices 20 and 39. At the same time, the new setpoints are stored in memory for use the next time the DC-3 program is executed. In that manner, the headbox consistency control program DC-3 computes slice position setpoint signals for the headboxes based on the errors between the headbox flow setpoints (computed in accordance with the foregoing equations 16 and i7 and the computed flows of the headboxes in accordance with the foregoing equations 14 and 15. A separate slice control program may be called up for execution as part of the step of transmitting new setpoint signals to slice position controllers 46 and 47 depending upon the configuration of the controllers.

The effect of adjusting the position of the slices 20 and 39 is to compensate for variations in headbox con sistency by computing headbox total flow setpoints based on fiber flow rates and headbox consistency setpoints SPBLHBC and SPTLHBO entered by the operator. The slice position is normally used as a control of rush or drag, which is the relative velocity with which stock is delivered to the wire screen, rush being a positive relative velocity and drag being a negative relative velocity. However, in accordance with the present invention, the rush or drag desired is established instead by controlling the head (pressure) in the headboxes through operation of section DC-4.'

DC-4 SECTION The total head of each of the headboxes l2 and 32 is controlled to maintain a specified rush or drag, which may be more aptly defined as the ratio of the slice jet speed to the wire speed. The program of the computer section DC-4 accomplishes that by monitoring wire speed with sensor 41 and periodically (every 15 seconds) computing from wire speed a theoretical head THHD as shown for the first step in the flow chart of FIG. 6 in accordance with the following equation:

THHD=SPEED K (32) where K is a conversion factor equal to kg. Following that in the second step, a rush-drag setpoint is added to the theoretical head in terms of positive or negative inches of head in accordance with the following equation:

SPBLHD=THHD+BLRDlN 33) where SPBLHD is a bottom layer head setpoint signal transmitted to head controller 50, and BLRDIN is the desired rush or drag in inches of head (positive for rush, negative for drag).

In some paper machine installations, it is customary to specify the rush or drag as a ratio by which the theoretical head is to be multiplied, the ratio being greater than one for rush and less than one for drag. Where that is the case, the control equation for the bottom layer head setpoint may be readily modified accordingly.

The error in the bottom layer head is then computed in accordance with the following equation:

ERBLHD=SPBLHDBLHD (34) where BLHD is the bottom layer head measured by a suitable transducer and transmitted to the section DC- 4. However, before that is done, the previous error in bottom level head is transmitted to a specified memory location for use in a two-mode control algorithm of the following fo'rm:

BLHDIN=A ERBLHD +B PERBLHD 35 where A and B are constants tuned for a bottom layer headbox control. The bottom layer head increment BLHDIN is then transmitted to the controller 50, but first the same computations are made for the top layer headbox 32 to derive a control output TLHDIN transmitted to a headbox controller 51. In that manner, rush or drag is controlled by controlling the pressure in the headboxes l2 and 32, while consistency is controlled independently by controlling the positions of the slices and 39.

ln some older paper mills, open headboxes are employed in which the level of stock alone determines the head, instead of level and air pressure above the stock in the headbox as in the present illustrative embodiment of the present invention. Consequently, in such older mills, the signals BLHD and TLHD would be indicative of the present stock levels in the headboxes l2 and 32, and the control signals would be level setpoints to controllers l4 and 34.

Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently it is intended that the claims be interpreted to cover such modifications and equivalents.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A paper machine including apparatus for controlling stock flow to a headbox in order to anticipate the effects of changes in variables of the machine on the basis weight of paper being produced comprising:

first means for detecting changes in the stock consistency of stock flowing to said headbox; and

second means for increasing the rate of stock flow to said headbox for a decrease in consistency and decreasing the stock flow to said headbox for an increase in consistency.

2. Apparatus as defined in claim 1 wherein said first means additionally detects changes in machine speed and said second means additionally operates to increase the stock flow for an increase in speed and decrease the stock flow for a decrease in speed.

3. Apparatus as defined in claim 2 wherein the increases and decreases in stock flow in response to changes in machine speed are in increments, each proportional to a ratio of change in speed since a previous incremental change was made on stock flow to present speed.

4. Apparatus as defined in claim 1 wherein said increases and decreases are in increments, each proportional to a ratio of change in consistency since a previous incremental change was made on a stock flow to present consistency.

5. Apparatus as defined in claim 2 wherein said increases and decreases are in increments, each proportional to the difference between a ratio of change in speed since a previous incremental change was made on stock flow to present speed and a ratio of change in consistency since a previous incremental change was made on stock flow to present consistency.

6. In a machine for producing paper with two layers of fiber deposited on a wire screen from separate headboxes, apparatus for controlling stock flowsto each headbox in order to maintain a desired composite dry. basis weight in paper being produced with a desired dry basis weight of one layer comprising:

first means for measuring the total basis weight and moisture of paper leaving said machine and producing a signal representing each measurement;

second means responsive to signals from said first means for computing composite dry basis weight by subtracting said moisture measurement from said total basis weight measurement;

third means for measuring the rate of flow and consistency of stock furnished for use in producing said one layer of paper through one headbox and for producing a signal representing each measurement;

fourth means for measuring the rate of flow and consistency of stock furnished for use in producing the other layer of paper through the other headbox and for producing a signal representing each measurement;

fifth means responsive to signals from said first and third means for computing the dry basis weight of said one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of measurements made by said third means to the sum of the product of measurements made by said fourth means and the product of measurements made by said third means;

sixth means for computing the difference between computed composite dry basis weight and desired composite dry basis weight;

seventh means for computing the difference between computed dry basis weight and desired dry basis weight of said one layer;

eighth means for controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint;

ninth means for computing a change in said setpoint for said eighth means from a ratio of said change to the present rate of flow measured by said third means proportionate to the ratio of the difference computed by said seventh means to the dry basis weight of said one layer computed by said fifth means;

tenth means for controlling flow of stock for use in producing the other layer of paper at a predetermined setpoint; and

eleventh means for computing a change in said setpoint for said tenth means from a ratio of said change to the present rate of flow measured by said fourth means proportionate to the ratio of the difference computed by said sixth means to the computed composite dry basis weight, and from the computed change subtracting a fraction of said change computed by said ninth means where said fraction is the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper.

7. Apparatus as defined in claim 6 further comprising computational means for computing further incremental changes in said setpoints for said eighth and tenth means as a function of certain changes in variables of the machine which affect basis weight, and means for I detecting said changes in variables and in response thereto transmitting to said computational means signals representing said changes in variables.

8. Apparatus as defined in claim 7 wherein said certain changes in variables include changes in machine speed.

9. Apparatus as defined in claim 8 wherein said further incremental changes in said setpoints are pro portional to a ratio of change in speed to present speed.

10. Apparatus as defined in claim 9 wherein said certain changes in variables include stock consistency.

11. Apparatus as defined in claim 10 wherein said further incremental changes in said setpoints are proportional to a ratio of changes in consistency to present consistency of stock being controlled as to flow rate.

12. Apparatus for controlling the slice position of a headbox in a paper machine to compensate for variations in the consistency of stock flowing therethrough comprising:

first means for measuring the rate of flow BLFL and consistency BLC of thick stock furnished to said headbox and producing a signal representing each measurement; second means for determining headbox flow BLHBF of stock through said slice and in response thereto producing a signal representing headbox flow;

third means responsive to signals from said first means for computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC and a predetermined headbox consistency setpoint SPBLHBC;

fourth means responsive to signals from said second means and said computed setpoint SPBLHBF for determining any error ERBLHBF in headbox flow as the difference between said setpoint SPBLHBF and said headbox flow signal BLHBF;

fifth means for determining present slice position BLSL and in response thereto producing a signal representing present slice position;

sixth means responsive to said fifth means for computing a slice position increment BLSPI from the present slice BLSL position as a function of said error ERBLHBF and developing a signal representative of the position increment BLSPI; and seventh means responsive to said signal developed by said sixth means for controlling the position of said slice to a position specified by the sum of a predetermined setpoint SPBLSL and said signal representing said slice position increment.

13. Apparatus as defined in claim 12 wherein said sixth means computes said slice position increment in accordance with a two-mode algorithm.

14. Apparatus as defined in claim 13 wherein said algorithm is digital in accordance with the following equation:

BLSPI BLSL (G ERBLHBF/BLHBF H PERBLHBF/BLHBF) where PERBLHBF is a previous error ERBLHBF.

15. Apparatus as defined in claim 12 including apparatus for controlling the jet speed of stock from the headbox slice relative to the speed of a wire screen to maintain a desired difference comprising:

first means for measuring the speed of said wire screen and the head of stock over said slice and producing a signal representing each measurement;

second means responsive to said first means for computing a theoretical head necessary to produce a jet speed equal to said wire screen speed as represented by a signal produced by said first means; third means for adding to said theoretical head an increment equal to that necessary to produce said 5 desired difference in speeds of said jet and said wire screen;

fourth means responsive to said first and third means for determining the difference between the sum computed by said third means and the head as represented by a signal produced by said first means and developing a signal representing said difference; and

fifth means responsive to said difference signal produced by said fourth means for controlling the head of stock over said slice to a new magnitude in accordance therewith.

16. Apparatus for controlling the jet speed of stock from a headbox slice relative to the speed of a wire 20 screen in a paper machine to maintain a desired difference comprising:

first means for measuring the speed of said wire screen and the head of stock over said slice and producing a signal representing each measurement;

second means responsive to said first means for computing a theoretical head necessary to produce a jet speed equal to said wire screen speed as represented by a signal produced by said first means;

third means for adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen;

fourth means responsive to said first and third means for determining the difference between the sum computed by said third means and the head as represented by a signal produced by said first means and producing a signal representing said difference; and

fifth means responsive to said difference signal produced by said fourth means for controlling the head of stock over said slice to a new magnitude in accordance therewith.

17. Apparatus as defined in claim 16 wherein said fourth means controls said head in accordance with a twomode algorithm.

18. Apparatus as defined in claim 17 wherein said algorithm is digital in accordance with the following equation:

TLHDIN A ERBLHD B PERBLHD where all terms are in digital form, A and B are constants, PERBLHD is the previous difference in head determined by said fourth means. ERBLHD is the present difference in head determined by said fourth means, and TLHDIN is an incremental change to be made in a setpoint for a head controller.

19. in a machine for producing paper having two layers, apparatus for maintaining the dry basis weight of one layer at a desired value and for maintaining the composite dry basis weight of both layers at a desired level comprising:

first means for measuring total basis weight and moisture content of said paper;

second means for computing the composite dry basis weight of said paper from measurementsmade by said first means;

third means for measuring the flow rate and consistency of stock for each of said layers;

fourth means for computing from measurements made by said third means flow rates of fiber to said layers; fifth means for computing the dry basis weight of one of said layers as the product of said composite dry basis weight and the ratio of the flow rate of fiber to said one of said layers as computed by said fourth means to the sum of the flow rates of fiber to both layers as computed by said fourth means;

sixth means for correcting the dry basis weight of said one layer as computed by said fifth means to said desired level by adjusting the flow of stock to said one layer as a function of the difference between its computed dry basis weight and said desired level; and

seventh means for correcting the composite dry basis weight by adjusting the flow of stock to the other layer as a function of the difference between the composite dry basis weight computed by said second means and the desired composite dry basis weight taking into consideration the change in the flow of stock being made to the one layer by said sixth means.

20. Apparatus as described in claim 19 further comprising means for anticipating a change in said computed composite dry basis weight resulting from a change in said machine operation by adjusting the flow of stock to said layers to compensate for said change in machine operation in proportion to the flow rate of stock being furnished to each layer.

21. Apparatus as described in claim 20 wherein said change in machine operation is a change in speed and the flow rate of stock being furnished to each layer is changed by an increment computed from a ratio of such increment to the present flow rate proportionate to a ratio of the change in speed to the present speed, where both ratios have the same sign.

22. Apparatus as described in claim 20 further comprising means for anticipating a change in said computed composite dry basis weight resulting from a change in stock consistency furnished to each of said layers by adjusting the flow of stock to said layers to compensate for said change in consistency of each in proportion to its present flow rate.

23. Apparatus as described in claim 22 wherein the flow rate of stock to a given layer is changed by an increment computed from a ratio of such increment to the present flow rate proportionate to a ratio of the change in consistency to the present consistency, where the ratios have opposite signs.

24. in a machine for producing paper'with two layers of fiber deposited on a wire screen from separate headboxes, apparatus for controlling stock. flow to each headbox in order to maintain a desired composite dry basis weight in paper being produced with a desireddry basis weight of one layer, comprising:

computational means for receiving signals representing variables measured by sensors connected to said machine and signals representing predetermined quantities, and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine:

first means for measuring the total basis weight and moisture of paper leaving said machine and for transmitting to said computational means signals.

representing the total basis weight and moisture measurements, said computational means receiving said total basis weight and moisture signals for computing composite dry basis weight by subtracting the value of said moisture signal from the value of said total basis weight signal;

second means for measuring the rate of flow and consistency of stock furnished to each of two headboxes for use in producing separate layers of said paper and for transmitting to said computational means signals representing said measurements, said computational means receiving said flow and consistency signals for computing the dry basis weight of one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of measurements made by said second means for stock furnished to one of said headboxes associated with said one layer to the sum of the product of flow and consistency measurements made by said second means with respect to said one headbox and the product of flow and consistency measurements made by said second means with respect to the other of said two headboxes;

third means for entering into said computational means a signal representing a desired composite dry basis weight and a signal representing a desired dry basis weight of said one layer, said computational means receiving said desired composite dry basis weight signal and said desired dry basis a weight signal of said one layer for computing the difference between said computed composite dry basis weight and said desired composite dry basis weight in addition to computing the difference between said computed dry basis weight anddesired dry basis weight of said one layer;

fourth means for controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint;

fifth means for entering into said computational means a signal representing said setpoint for said fourth means, said computational means receiving said signal representing said predetermined setpoint for said fourth means and computinga change in said predetermined setpoint for said fourth means from a ratio of said change to the present rate of flow of stock to said headbox associated with said one layer as measured by said second means, said ratio bcingproportionate to the ratio of the difference between computed dry basis weight and desired dry basis weight of said one layer to the dry basis weight of said one layer,

said computational means transmitting to said.

fourth means a control signal representing a new setpoint computed from said predetermined setpoint therefor and the computed change in said setpoint, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said fourth means;

sixth means for controlling flow of stock for use in producing the other layer of paper at a predetermined setpoint; and

seventh means for entering into said computational means a signal representing said predetermined setpoint for said sixth means, said computational means receiving said signal representing a predetermined setpoint for said sixth means and computing a change therein from a ratio of said change to the present rate of flow of stock furnished for use in producing the other layer of paper measured by said second means, said ratio being proportionate to the ratio of the difference between computed composite dry basis weight and desired composite dry basis weight to the computed composite dry basis weight, and from the resulting computed change subtracting a fraction of said change computed for the setpoint of said fourth means, said fraction being the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper, said computational means transmitting to said sixth means a control signal representing a new setpoint computed from said predetermined setpoint therefor and the computed change in said setpoint, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said sixth means.

25. Apparatus as defined in claim 24 further comprising means for measuring variables in said machine which affect basis weight and for transmitting to said computational means signals representing said variables, said computational means computing a further incremental change in said setpoints associated with said fourth and sixth means as a function of change in said variables of the machine.

26. Apparatus as defined in claim 25 wherein said further incremental changes in said setpoints are proportional to a ratio of change in speed to present speed.

27. Apparatus as defined in claim 25 wherein said certain changes in variables include stock consistency as measured by said second means.

28. Apparatus as defined in claim 27 wherein said further incremental changes in said setpoints are proportional to a ratio of changes in consistency to present consistency of stock being controlled as to flow rate.

29. In a machine for producing paper, apparatus for controlling the slice position of a headbox to compensate for variations in the consistency of stock. flowing therethrough, comprising:

computational means for receiving signals representing variables measured by sensors connected to said machine, and other values entered therein, and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine; first means for measuring the rate of flow BLFL-and consistency BLC of thick stock furnished to said headbox, and for transmitting a signal representing said flow measurement BLFL to said computational means, said computational means computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC, and a predetermined headbox consistency setpoint SPBLHBC entered into said computational means as a signal representing the desired headbox consistency; second means for measuring variables from which headbox flow BLHBF may be determined and for transmitting to said computational means signals representing said measured variables from which said headbox flow BLHBF may be determined, said computational means being responsive, to said signals representing variables from which headbox flow BLHBF may be determined, for determining headbox flow BLHBF, said computational means further determining an error ERBLHBF in headbox flow as the difference between said computed setpoint SPBLHBF and said determined headbox flow BLHBF; third means for controlling the position of said slice to a position specified by a predetermined setpoint SPBLSL;

fourth means for measuring present slice position BLSL and for transmitting to said computational means a signal representing said present slice position BLSL, said computational means receiving said signal representing present slice position BLSL and in response thereto computing a slice position increment BLSPl from the present slice position BLSL as a function of said error ERBLHBF; and

fifth means for entering into said computational means said predetermined setpoint SPBLSL, said computational means receiving said signal representing said predetermined setpoint SPBLSL and in response thereto transmitting to said third means a new setpoint for controlling the position of said slice to a position specified by the sum of said predetermined setpoint SPBLSL and said slice position increment BLSPl, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said third means.

30. Apparatus as defined in claim 29 wherein said computational means computes said slice position increment BLSPl in accordance with the following twomode algorithm equation:

BLSPI BLSL (G ERBLHBF/BLHBF H PERBLHBF/BLHBF) where PERBLHBF is a previous error ERBLHBF.

31. Apparatus as defined in claim 30 including: means for controlling the head of stock over said slice to control the jet speed of stock from the slice relative to the speed of a wire screen; first means for measuring the speed of said wire screen and the head of stock over said slice, and for transmitting to said computational means signals representing speed and head measurements, said computational means receiving said speed and head measurement signals and in response thereto computing a theoretical head necessary to produce a jet speed equal to said w'ire screen speed, adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen, and for determining the difference between the sum just computed and the head represented by a signal produced by said first means for transmitting a signal representing said difference to control means, said control means being responsive to said difference signal for controlling the head of stock over said slice in accordance with the algebraic sum of said computed difference and the measured head of stock.

32. In a machine for producing paper, apparatus for controlling the jet speed of stock from a headbox slice relative to the speed of a wire screen in a paper machine to maintain a desired speed difference comprising:

computational means for receiving signals representing variables measured by sensors connected to said machine and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine;

means for measuring the speed of said wire screen and a head of stock over said slice, and for transmitting to said computational means signals representing speed of said wire screen and the head of stock over said slice, said computational means computing a theoretical head necessary to produce a jet speed equal to said wire screen speed and adding to said theoretical head an increment equal to that necessary to produce said desired difference in speed of said jet and said wire screen, said computational means further determining the difference between said computed sum and the measured head, said computational means transmitting a signal representing the difference between said computed sum and said measured head; and

control means receiving said signal transmitted by said computational means and in response thereto controlling the head of stock over said slice to a new magnitude in accordance therewith.

33. In a continuous process for producing paper, controlling stock flow to a headbox in order to anticipate the effects of changes of variables of the machine on the basis weight of paper being produced comprising the steps of:

detecting changes in machine speed and stock consistency; and

changing the rate of stock flow to said headbox as a direct function of the detected speed and as an inverse function of the detected consistency.

34. in a process for producing paper, the invention as defined in claim 33 wherein the step of changing is such that changes occur in increments, each proportional to the difference between a ratio of change in speed since a previous incremental change was made on stock flow to present speed and a ratio of change in consistency since a previous incremental change was made on stock flow to present consistency.

35. In a process for producing paper with two layers of fiber deposited on a wire screen from separate headboxes, steps for controlling stock flow to each headbox in order to maintain a desired composite dry basis weight in paper being produced with a desired dry basis weight of one layer comprising:

measuring the total basis weight and moistureof paper leaving said machine;

computing composite dry basis weight by subtracting said moisture measurement from said total basis weight measurement;

measuring the rate of flow of consistency of stock furnished for use .in producing said one layer of paper through said headbox;

measuring the rate of flow and consistency of stock furnished for use in producing the other layer of paper through the other headbox;

computing the dry basis weight of said one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of the rate of flow and the consistency of stock furnished for use in producing said one layer of paper to the sum of the product of the rate of flow and consistency of vstock furnished for use in producing the other layer of paper and the product of the rate of flow and consistency of stock furnished for use in producing said one layer of paper;

computing the difference between computed composite dry basis weight and desired composite dry basis weight;

computing the difference between computed dry basis weight and desired dry basis weight of said one layer;

controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint;

computing a change in said setpoint from a ratio of said change to the present rate of flow of stock furnished for use in producing said one layer of paper proportionate to the ratio of the difference between computed dry basis weight and desired basis weight of said one layer to the dry basis weight of said one layer computed from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of the rate of flow and consistency of stock furnished for use in producing said one layer to the sum of the product of the rate of flow and consistency of stock furnished for use in producing the other layer of paper and the product of the rate of flow and consistency of stock furnished for use in producing said one layer of paper;

controlling the flow of stock for use in producing the other layer of paper at a predetermined setpoint; and

computing a change in said setpoint for the control of stock flow in the production of the other layer of paper from a ratio of said change to the present rate of flow of stock furnished for use in producing the other layer of paper proportionate to the ratio of the difference between computed composite dry basis weight and desired composite basis weight to the computed composite dry basis weight, and from the computed change subtracting a fraction of said computed change in the setpoint for control of stock in the production of said one layer where said fraction is the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper.

36. ln a process for producing paper as defined in claim 35, computing further incremental changes in said setpoints for control of stock flow in the production of said one layer of paper and said other layer of paper as a function of certain changes in variables of the machine which affect basis weight, said further incremental changes being proportional to a ratio of said certain changes to present values of said variables.

37. ln a process for producing paper, steps for controlling the slice position of a headbox in a paper machine to compensate for variations in the consistency of stock flowing therethrough comprising:

measuring the rate of flow BLFL and consistency BLC of thick stock furnished to said headbox; determining headbox flow BLHBF of stock through said slice;

computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC and a predetermined headbox consistency setpoint SPBLHBC;

determining an error ERBLHBF in headbox flow as the difference between said computed setpoint SPBLHBF and said headbox flow BLHBF; determining present slice position BLSL;

computing a slice position increment BLSPl from the present slice BLSL position as a function of said error ERBLHBF; and controlling the position of said slice to a position specified by the sum of a predetermined setpoint SPBLSL and said slice position increment BLSPl. 38. In a process for producing paper, steps for controlling jet speed of stock from a headbox slice relative to the speed of a wire screen in a paper machine to maintain a desired difference comprising:

measuring the speed of said wire screen and the head of said stock over said slice; computing a theoretical head necessary to produce a jet speed equal to said wire screen speed; adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen; determining the difference between the sum computed in the preceding step and the measured head; and controlling the head of stock over said slice to a new magnitude in accordance with the difference computed in the preceding step. 

1. A paper machine including apparatus for controlling stock flow to a headbox in order to anticipate the effects of changes in variables of the machine on the basis weight of paper being produced comprising: first means for detecting changes in the stock consistency of stock flowing to said headbox; and second means for increasing the rate of stock flow to said headbox for a decrease in consistency and decreasing the stock flow to said headbox for an increase in consistency.
 2. Apparatus as defined in claim 1 wherein said first means additionally detects changes in machine speed and said second means additionally operates to increase the stock flow for an increase in speed and decrease the stock flow for a decrease in speed.
 3. Apparatus as defined in claim 2 wherein the increases and decreases in stock flow in response to changes in machine speed are in increments, each proportional to a ratio of change in speed since a previous incremental change was made on stock flow to present speed.
 4. Apparatus as defined in claim 1 wherein said increases and decreases are in increments, each proportional to a ratio of change in consistency since a previous incremental change was made on a stock flow to present consistency.
 5. Apparatus as defined in claim 2 wherein said increases and decreases are in increments, each proportional to the difference between a ratio of change in speed since a previous incremental change was made on stock flow to present speed and a ratio of change in consistency since a previous incremental change was made on stock flow to present consistency.
 6. In a machine for producing paper with two layers of fiber deposited on a wire screen from separate headboxes, apparatus for controlling stock flow to each headbox in order to maintain a desired composite dry basis weight in paper being produced with a desired dry basis weight of one layer comprising: first means for measuring the total basis weight and moisture of paper leaving said machine and producing a signal representing each measurement; second means responsive to signals from said first means for computing composite dry basis weight by subtracting said moisture measurement from said total basis weight measurement; third means for measuring the rate of flow and consistency of stock furnished for use in producing said one layer of paper through one headbox and for producing a signal representing each measurement; fourth means for measuring the rate of flow and consistency of stock furnished for use in producing the other layer of paper through the other headbox and for producing a signal representing each measurement; fifth means responsive to signals from said first and third means for computing the dry basis weight of said one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of measurements made by said third means to the sum of the product of measurements made by said fourth means and the product of measurements made by said third means; sixth means for computing the difference between computed composite dry basis weight and desired composite dry basis weight; seventh means for computing the difference between computed dry basis weight and desired dry basis weight of said one layer; eighth means for controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint; ninth means for computing a change in said setpoint for said eighth means from a ratio of said change to the present rate of flow measured by said third means proportionate to the ratio of the difference computed by said seventh means to the dry basis weight of said one layer computed by said fifth means; tenth means for controlling flow of stock for use in producing the other layer of paper at a predetermined setpoint; and eleventh means for computing a change in said setpoint for said tenth means from a ratio of said change to the present rate of flow measured by said fourth means proportionate to the ratio of the difference computed by said sixth means to the computed composite dry basis weight, and from the computed change subtracting a fraction of said change computed by said ninth means where said fraction is the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper.
 7. Apparatus as defined in claim 6 further comprising computational means for computing further incremental changes in said setpoints for said eighth and tenth means as a function of certain changes in variables of the machine which affect basis weight, and means for detecting said changes in variables and in response thereto transmitting to said computational means signals representing said changes in variables.
 8. Apparatus as defined in claim 7 wherein said certain changes in variables include changes in machine speed.
 9. Apparatus as defined in claim 8 wherein said further incremental changes in said setpoints are proportional to a ratio of change in speed to present speed.
 10. Apparatus as defined in claim 9 wherein said certain changes in variables include stock consistency.
 11. Apparatus as defined in claim 10 wherein said further incremental changes in said setpoints are proportional to a ratio of changes in consistency to present consistency of stock being controlled as to flow rate.
 12. Apparatus for controlling the slice position of a headbox in a paper machine to compensate for variations in the consistency of stock flowing therethrough comprising: first means for measuring the rate of flow BLFL and consistency BLC of thick stock furnished to said headbox and producing a signal representing each measurement; second means for determining headbox flow BLHBF of stock through said slice and in response thereto producing a signal representing headbox flow; third means responsive to signals from said first means for computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC and a predetermined headbox consistency setpoint SPBLHBC; fourth means responsive to signals from said second means and said computed setpoint SPBLHBF for determining any error ERBLHBF in headbox flow as the difference between said setpoint SPBLHBF and said headbox flow signal BLHBF; fifth means for determining present slice position BLSL and in response thereto producing a signal representing present slice position; sixth means responsive to said fifth means for computing a slice position increment BLSPI from the present slice BLSL position as a function of said error ERBLHBF and developing a signal representative of the position increment BLSPI; and seventh means responsive to said signal developed by said sixth means for controlling the position of said slice to a position specified by the sum of a predetermined setpoint SPBLSL and said signal representing said slice position increment.
 13. Apparatus as defined in claim 12 wherein said sixth means computes said slice position increment in accordance with a two-mode algorithm.
 14. Apparatus as defined in claim 13 wherein said algorithm is digital in accordance with the following equation: BLSPI BLSL (G ERBLHBF/BLHBF + H PERBLHBF/BLHBF) where PERBLHBF is a previous error ERBLHBF.
 15. Apparatus as defined in claim 12 including apparatus for controlling the jet speed of stock from the headbox slice relative to the speed of a wire screen to maintain a desired difference comprising: first means for measuring the speed of said wire screen and the head of stock over said slice and producing a signal representing each measurement; second means responsive to said first means for computing a theoretical head necessary to produce a jet speed equal to said wire screen speed as represented by a signal produced by said first means; third means for adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen; fourth means responsive to said first and third means for determining the difference between the sum computed by said third means and the head as represented by a signal produced by said first means and developing a signal representing said difference; and fifth means responsive to said difference signal produced by said fourth means for controlling the head of stock over said slice to a new magnitude in accordance therewith.
 16. Apparatus for controlling the jet speed of stock from a headbox slice relative to the speed of a wire screen in a paper machine to maintain a desired difference comprising: first means for measuring the speed of said wire screen and the head of stock over said slice and producing a signal representing each measurement; second means responsive to said first means for computing a theoretical head necessary to produce a jet speed equal to said wire screen speed as represented by a signal produced by said first means; third means for adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen; fourth means responsive to said first and third means for determining the difference between the sum computed by said third means and the head as represented by a signal produced by said first means and producing a signal representing said difference; and fifth means responsive to said difference signal produced by said fourth means for controlling the head of stock over said slice to a new magnitude in accordance therewith.
 17. Apparatus as defined in claim 16 wherein said fourth means controls said head in accordance with a two-mode algorithm.
 18. Apparatus as defined in claim 17 wherein said algorithm is digital in accordance with the following equation: TLHDIN A . ERBLHD + B . PERBLHD where all terms are in digital form, A and B are constants, PERBLHD is the previous difference in head determined by said fourth means, ERBLHD is the present difference in head determined by said fourth means, and TLHDIN is an incremental change to be made in a setpoint for a head controller.
 19. In a machine for producing paper having two layers, apparatus for maintaining the dry basis weight of one layer at a desired value and for maintaining the composite dry basis weight of both layers at a desired level comprising: first means for measuring total basis weight and moisture content of said paper; second means for computing the composite dry basis weight of said paper from measurements made by said first means; third means for measuring the flow rate and consistency of stock for Each of said layers; fourth means for computing from measurements made by said third means flow rates of fiber to said layers; fifth means for computing the dry basis weight of one of said layers as the product of said composite dry basis weight and the ratio of the flow rate of fiber to said one of said layers as computed by said fourth means to the sum of the flow rates of fiber to both layers as computed by said fourth means; sixth means for correcting the dry basis weight of said one layer as computed by said fifth means to said desired level by adjusting the flow of stock to said one layer as a function of the difference between its computed dry basis weight and said desired level; and seventh means for correcting the composite dry basis weight by adjusting the flow of stock to the other layer as a function of the difference between the composite dry basis weight computed by said second means and the desired composite dry basis weight taking into consideration the change in the flow of stock being made to the one layer by said sixth means.
 20. Apparatus as described in claim 19 further comprising means for anticipating a change in said computed composite dry basis weight resulting from a change in said machine operation by adjusting the flow of stock to said layers to compensate for said change in machine operation in proportion to the flow rate of stock being furnished to each layer.
 21. Apparatus as described in claim 20 wherein said change in machine operation is a change in speed and the flow rate of stock being furnished to each layer is changed by an increment computed from a ratio of such increment to the present flow rate proportionate to a ratio of the change in speed to the present speed, where both ratios have the same sign.
 22. Apparatus as described in claim 20 further comprising means for anticipating a change in said computed composite dry basis weight resulting from a change in stock consistency furnished to each of said layers by adjusting the flow of stock to said layers to compensate for said change in consistency of each in proportion to its present flow rate.
 23. Apparatus as described in claim 22 wherein the flow rate of stock to a given layer is changed by an increment computed from a ratio of such increment to the present flow rate proportionate to a ratio of the change in consistency to the present consistency, where the ratios have opposite signs.
 24. In a machine for producing paper with two layers of fiber deposited on a wire screen from separate headboxes, apparatus for controlling stock flow to each headbox in order to maintain a desired composite dry basis weight in paper being produced with a desired dry basis weight of one layer, comprising: computational means for receiving signals representing variables measured by sensors connected to said machine and signals representing predetermined quantities, and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine: first means for measuring the total basis weight and moisture of paper leaving said machine and for transmitting to said computational means signals representing the total basis weight and moisture measurements, said computational means receiving said total basis weight and moisture signals for computing composite dry basis weight by subtracting the value of said moisture signal from the value of said total basis weight signal; second means for measuring the rate of flow and consistency of stock furnished to each of two headboxes for use in producing separate layers of said paper and for transmitting to said computational means signals representing said measurements, said computational means receiving said flow and consistency signals for computing the dry basis weight of one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of measurements made by said second means for stock furnished to one of said headboxes assoCiated with said one layer to the sum of the product of flow and consistency measurements made by said second means with respect to said one headbox and the product of flow and consistency measurements made by said second means with respect to the other of said two headboxes; third means for entering into said computational means a signal representing a desired composite dry basis weight and a signal representing a desired dry basis weight of said one layer, said computational means receiving said desired composite dry basis weight signal and said desired dry basis weight signal of said one layer for computing the difference between said computed composite dry basis weight and said desired composite dry basis weight in addition to computing the difference between said computed dry basis weight and desired dry basis weight of said one layer; fourth means for controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint; fifth means for entering into said computational means a signal representing said setpoint for said fourth means, said computational means receiving said signal representing said predetermined setpoint for said fourth means and computing a change in said predetermined setpoint for said fourth means from a ratio of said change to the present rate of flow of stock to said headbox associated with said one layer as measured by said second means, said ratio being proportionate to the ratio of the difference between computed dry basis weight and desired dry basis weight of said one layer to the dry basis weight of said one layer, said computational means transmitting to said fourth means a control signal representing a new setpoint computed from said predetermined setpoint therefor and the computed change in said setpoint, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said fourth means; sixth means for controlling flow of stock for use in producing the other layer of paper at a predetermined setpoint; and seventh means for entering into said computational means a signal representing said predetermined setpoint for said sixth means, said computational means receiving said signal representing a predetermined setpoint for said sixth means and computing a change therein from a ratio of said change to the present rate of flow of stock furnished for use in producing the other layer of paper measured by said second means, said ratio being proportionate to the ratio of the difference between computed composite dry basis weight and desired composite dry basis weight to the computed composite dry basis weight, and from the resulting computed change subtracting a fraction of said change computed for the setpoint of said fourth means, said fraction being the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper, said computational means transmitting to said sixth means a control signal representing a new setpoint computed from said predetermined setpoint therefor and the computed change in said setpoint, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said sixth means.
 25. Apparatus as defined in claim 24 further comprising means for measuring variables in said machine which affect basis weight and for transmitting to said computational means signals representing said variables, said computational means computing a further incremental change in said setpoints associated with said fourth and sixth means as a function of change in said variables of the machine.
 26. Apparatus as defined in claim 25 wherein said further incremental changes in said setpoints are proportional to a ratio of changE in speed to present speed.
 27. Apparatus as defined in claim 25 wherein said certain changes in variables include stock consistency as measured by said second means.
 28. Apparatus as defined in claim 27 wherein said further incremental changes in said setpoints are proportional to a ratio of changes in consistency to present consistency of stock being controlled as to flow rate.
 29. In a machine for producing paper, apparatus for controlling the slice position of a headbox to compensate for variations in the consistency of stock flowing therethrough, comprising: computational means for receiving signals representing variables measured by sensors connected to said machine, and other values entered therein, and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine; first means for measuring the rate of flow BLFL and consistency BLC of thick stock furnished to said headbox, and for transmitting a signal representing said flow measurement BLFL to said computational means, said computational means computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC, and a predetermined headbox consistency setpoint SPBLHBC entered into said computational means as a signal representing the desired headbox consistency; second means for measuring variables from which headbox flow BLHBF may be determined and for transmitting to said computational means signals representing said measured variables from which said headbox flow BLHBF may be determined, said computational means being responsive, to said signals representing variables from which headbox flow BLHBF may be determined, for determining headbox flow BLHBF, said computational means further determining an error ERBLHBF in headbox flow as the difference between said computed setpoint SPBLHBF and said determined headbox flow BLHBF; third means for controlling the position of said slice to a position specified by a predetermined setpoint SPBLSL; fourth means for measuring present slice position BLSL and for transmitting to said computational means a signal representing said present slice position BLSL, said computational means receiving said signal representing present slice position BLSL and in response thereto computing a slice position increment BLSPI from the present slice position BLSL as a function of said error ERBLHBF; and fifth means for entering into said computational means said predetermined setpoint SPBLSL, said computational means receiving said signal representing said predetermined setpoint SPBLSL and in response thereto transmitting to said third means a new setpoint for controlling the position of said slice to a position specified by the sum of said predetermined setpoint SPBLSL and said slice position increment BLSPI, and said computational means substituting said new setpoint for said predetermined setpoint for use in a subsequent iteration of computational steps leading to a new control signal to be transmitted to said third means.
 30. Apparatus as defined in claim 29 wherein said computational means computes said slice position increment BLSPI in accordance with the following two-mode algorithm equation: BLSPI BLSL (G ERBLHBF/BLHBF + H PERBLHBF/BLHBF) where PERBLHBF is a previous error ERBLHBF.
 31. Apparatus as defined in claim 30 including: means for controlling the head of stock over said slice to control the jet speed of stock from the slice relative to the speed of a wire screen; first means for measuring the speed of said wire screen and the head of stock over said slice, and for transmitting to said computational means signals representing speed and head measurements, said computational means receiving said speed and head measurement signals and in response thereto computing a theoretical head necessary to produce A jet speed equal to said wire screen speed, adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen, and for determining the difference between the sum just computed and the head represented by a signal produced by said first means for transmitting a signal representing said difference to control means, said control means being responsive to said difference signal for controlling the head of stock over said slice in accordance with the algebraic sum of said computed difference and the measured head of stock.
 32. In a machine for producing paper, apparatus for controlling the jet speed of stock from a headbox slice relative to the speed of a wire screen in a paper machine to maintain a desired speed difference comprising: computational means for receiving signals representing variables measured by sensors connected to said machine and in response thereto for carrying out computational steps resulting in a control signal transmitted to said machine; means for measuring the speed of said wire screen and a head of stock over said slice, and for transmitting to said computational means signals representing speed of said wire screen and the head of stock over said slice, said computational means computing a theoretical head necessary to produce a jet speed equal to said wire screen speed and adding to said theoretical head an increment equal to that necessary to produce said desired difference in speed of said jet and said wire screen, said computational means further determining the difference between said computed sum and the measured head, said computational means transmitting a signal representing the difference between said computed sum and said measured head; and control means receiving said signal transmitted by said computational means and in response thereto controlling the head of stock over said slice to a new magnitude in accordance therewith.
 33. In a continuous process for producing paper, controlling stock flow to a headbox in order to anticipate the effects of changes of variables of the machine on the basis weight of paper being produced comprising the steps of: detecting changes in machine speed and stock consistency; and changing the rate of stock flow to said headbox as a direct function of the detected speed and as an inverse function of the detected consistency.
 34. In a process for producing paper, the invention as defined in claim 33 wherein the step of changing is such that changes occur in increments, each proportional to the difference between a ratio of change in speed since a previous incremental change was made on stock flow to present speed and a ratio of change in consistency since a previous incremental change was made on stock flow to present consistency.
 35. In a process for producing paper with two layers of fiber deposited on a wire screen from separate headboxes, steps for controlling stock flow to each headbox in order to maintain a desired composite dry basis weight in paper being produced with a desired dry basis weight of one layer comprising: measuring the total basis weight and moisture of paper leaving said machine; computing composite dry basis weight by subtracting said moisture measurement from said total basis weight measurement; measuring the rate of flow of consistency of stock furnished for use in producing said one layer of paper through said headbox; measuring the rate of flow and consistency of stock furnished for use in producing the other layer of paper through the other headbox; computing the dry basis weight of said one layer from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of the rate of flow and the consistency of stock furnished for use in producing said one layer of paper to the sum of the product of the rate of flow and consistency of stock furnished for use in producing the other layer of paper and the pRoduct of the rate of flow and consistency of stock furnished for use in producing said one layer of paper; computing the difference between computed composite dry basis weight and desired composite dry basis weight; computing the difference between computed dry basis weight and desired dry basis weight of said one layer; controlling flow of stock for use in producing said one layer of paper at a predetermined setpoint; computing a change in said setpoint from a ratio of said change to the present rate of flow of stock furnished for use in producing said one layer of paper proportionate to the ratio of the difference between computed dry basis weight and desired basis weight of said one layer to the dry basis weight of said one layer computed from a ratio of its dry basis weight to said composite dry basis weight proportionate to a ratio of the product of the rate of flow and consistency of stock furnished for use in producing said one layer to the sum of the product of the rate of flow and consistency of stock furnished for use in producing the other layer of paper and the product of the rate of flow and consistency of stock furnished for use in producing said one layer of paper; controlling the flow of stock for use in producing the other layer of paper at a predetermined setpoint; and computing a change in said setpoint for the control of stock flow in the production of the other layer of paper from a ratio of said change to the present rate of flow of stock furnished for use in producing the other layer of paper proportionate to the ratio of the difference between computed composite dry basis weight and desired composite basis weight to the computed composite dry basis weight, and from the computed change subtracting a fraction of said computed change in the setpoint for control of stock in the production of said one layer where said fraction is the ratio of the consistency of stock furnished for use in producing said one layer of paper to the consistency of stock furnished for use in producing said other layer of paper.
 36. In a process for producing paper as defined in claim 35, computing further incremental changes in said setpoints for control of stock flow in the production of said one layer of paper and said other layer of paper as a function of certain changes in variables of the machine which affect basis weight, said further incremental changes being proportional to a ratio of said certain changes to present values of said variables.
 37. In a process for producing paper, steps for controlling the slice position of a headbox in a paper machine to compensate for variations in the consistency of stock flowing therethrough comprising: measuring the rate of flow BLFL and consistency BLC of thick stock furnished to said headbox; determining headbox flow BLHBF of stock through said slice; computing a setpoint SPBLHBF for the flow of dilute stock through said slice as a function of said measured rate of flow BLFL, said measured consistency BLC and a predetermined headbox consistency setpoint SPBLHBC; determining an error ERBLHBF in headbox flow as the difference between said computed setpoint SPBLHBF and said headbox flow BLHBF; determining present slice position BLSL; computing a slice position increment BLSPI from the present slice BLSL position as a function of said error ERBLHBF; and controlling the position of said slice to a position specified by the sum of a predetermined setpoint SPBLSL and said slice position increment BLSPI.
 38. In a process for producing paper, steps for controlling jet speed of stock from a headbox slice relative to the speed of a wire screen in a paper machine to maintain a desired difference comprising: measuring the speed of said wire screen and the head of said stock over said slice; computing a theoretical head necessary to produce a jet speed equal to said wire screen speed; adding to said theoretical head an increment equal to that necessary to produce said desired difference in speeds of said jet and said wire screen; determining the difference between the sum computed in the preceding step and the measured head; and controlling the head of stock over said slice to a new magnitude in accordance with the difference computed in the preceding step. 